The trends to reduce fuel consumption and emissions in internal combustion engine powered vehicles have placed increased demands on the performance of many components. Reducing fuel consumption by using lighter materials in engine cylinder blocks and head assemblies has proven successful, although the lighter alloys used typically experience greater deflection with equivalent cylinder compression ratios. This reduced stiffness may result in additional deflection within the head assembly and cylinder block, resulting in greater deflection between the head assembly and cylinder block, and thus, increased demand on a cylinder head gasket to accommodate relative deflection.
Reducing emissions by increasing the engine compression ratio has also proven successful. However, this increase in cylinder pressure typically results in increased motion between the mating surfaces of the head assembly and cylinder block. These factors, and others, have resulted in the technology of MLS cylinder head gaskets becoming an area of constant innovation.
The gasket areas immediately adjacent the circumference of engine cylinder bore apertures are subject to considerably greater stresses for assuring proper sealing than areas of the gasket radially remote from the apertures. These gasket areas immediately adjacent the circumference of engine cylinder bore apertures also experience greater displacement between the mating surfaces than areas of the gasket radially remote from the apertures. Typically, MLS gaskets incorporate at least one beaded region to ensure an adequate seal.
This displacement between the mating surfaces results in axial motion within the active layers and creates a micro-motion between the active layer and any adjoining surface. This motion requires a minimum durability in the beaded region to ensure that the gasket repeatably seals between the mating surfaces.
Layers with beaded regions, also called “active” layers, exert pressure on the sealing surfaces to ensure an adequate seal. Generally, the higher the surface pressures, the better the sealing function, or sealing capability. MLS gaskets sealing between a cylinder head assembly and a cylinder block are typically exposed to temperatures that often exceed 1600 degrees F.
Selection of materials for the active layers is typically limited due to the desired hardness, durability, spring rate, ductility, softening point, and other characteristics. Conventionally, materials have been selected by considering the available desirable properties of materials that have evolved into use.
A typical material for active layers of MLS gaskets is 301 stainless steel (301 SS). Tensile strength for 301 SS is in a range of about 1350 to 1600 MPA, and yield strength is in a range of about 1050 to 1250 MPA. While 301 SS has the capacity to retain adequate properties during engine operation, improved materials are sought to increase gasket sealability, reliability, and/or capacity to withstand increased displacement between the mating surfaces.